Diamond anvil cell behavior up to 4 Mbar Bing Lia,b,c,d,1, Cheng Jid,e, Wenge Yanga,d, Junyue Wanga,d, Ke Yangf, Ruqing Xug, Wenjun Liug, Zhonghou Caig, Jiuhua Chena,b,c, and Ho-kwang Maoa,d,h,1 aCenter for High Pressure Science and Technology Advanced Research, 201203 Shanghai, China; bCenter for the Study of Matter at Extreme Conditions, Florida International University, Miami, FL 33199; cDepartment of Mechanical and Materials Engineering, Florida International University, Miami, FL 33199; dHigh Pressure Synergetic Consortium, Carnegie Institution of Washington, Argonne, IL 60439; eHigh Pressure Collaborative Access Team, Geophysical Laboratory, Carnegie Institution of Washington, Argonne, IL 60439; fShanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, 201800 Shanghai, People’s Republic of China; gAdvanced Photon Source, Argonne National Laboratory, Argonne, IL 60439; and hGeophysical Laboratory, Carnegie Institution of Washington, Washington, DC 20015 Contributed by Ho-kwang Mao, January 12, 2018 (sent for review December 13, 2017; reviewed by Takehiko Yagi and Choong-Shik Yoo) The diamond anvil cell (DAC) is considered one of the dominant double cupping has not been observed experimentally at high devices to generate ultrahigh static pressure. The development of pressures up to 300 GPa. The experimental and theoretical dis- the DAC technique has enabled researchers to explore rich high- crepancy is likely due to either a low spatial resolution or not pressure science in the multimegabar pressure range. Here, we achieving high enough pressure in the experiments (18). investigated the behavior of the DAC up to 400 GPa, which is the In this paper, by using submicrometer focused synchrotron accepted pressure limit of a conventional DAC. By using a submi- X-ray beam, we performed in situ high-pressure synchrotron crometer synchrotron X-ray beam, double cuppings of the beveled X-ray diffraction and absorption experiments to investigate diamond anvils were observed experimentally. Details of pressure the behavior of the DAC up to 400 GPa. The submicrometer loading, distribution, gasket-thickness variation, and diamond anvil X-ray offers many advantages in high-pressure research (24, 25), deformation were studied to understand the generation of ultrahigh especially when dealing with small samples, because its high spatial pressures, which may improve the conventional DAC techniques. resolution makes it possible to resolve details within a tiny culet area. Mao-type symmetric DACs with culets of 20 μm beveled to high pressure | diamond anvil cell | deformation 250 or 200 μm with a beveled angle of 8.5° on Boehler-type seats (Fig. 1, Right) were used to achieve 400-GPa pressure. To simplify SCIENCES the study, an initial thickness of 250-μm tungsten (W) foil was used he diamond anvil cell (DAC) was invented almost 60 y ago APPLIED PHYSICAL T(1). It squeezes samples in between two opposing diamonds to as both the sample and gasket. Pressure was applied by tightening generate extremely high static pressure above 100 GPa (2). Many the screws, thus bending the spring washers and then applying a high-pressure DAC techniques have been developed to make it force proportional to the angles of the screw rotation. The central possible to generate pressures up to 400 GPa (3), such as the usage pressure as a function of the screw rotation was recorded in Fig. of a gasket, a beveled anvil design (4), different types of seat (5), etc. 2A (solid dots). This pressure-loading curve is nonlinear and can This makes the DAC a very important tool not only in geoscience, be divided into three sections with different loading rates. We will where it is used to study the earth’sinterioracrossthewhole discuss the loading curve in detail later. Fig. 2B presents a typical pressure range to the inner core ∼360 GPa, but also for exploring X-ray diffraction (XRD) pattern of W with a pressure of 170 GPa the novel phenomena of matter with extremely high density in at the center of the culet. The insert shows the W-(110) peak shift multidisciplinary scientific fields such as physics (6–8), chemistry (9– during compression, which indicates a pressure increase from 170 12), material science (13, 14), and even bioscience (15). Recently, to 398 GPa. an innovative use of a DAC with a pair of secondary nanodiamond The pressure distribution as a function of the radial distance across the culet is shown in Fig. 3 A and C at four selected center anvils was reported to achieve above 6 Mbar (16, 17). However, its ∼ ∼ ∼ ∼ technical challenge and difficult sample preparation have limited its pressures of 170, 240, 300, and 400 GPa, respectively. application as a routine way of generating high pressures. In conventional DAC experiments, typically flat and beveled Significance diamond anvils (Fig. 1, Left) are used to generate high pressures. For megabar pressure experiments (>100 GPa), beveled diamond Diamond anvil cell (DAC) could be one of the most popular and anvils with small culets are usually recommended in practice. versatile devices to generate static high pressure. By using Many factors can cause the failure of diamond anvils, such as the beveled anvils, researchers can study materials up to 4 Mbar, DAC alignment, DAC stability, anvil quality, gasket selection, etc. which is considered as a limit pressure of conventional DAC; Addressing all of these factors makes it challenging to achieve however, very few experiments have visited this pressure re- pressures above 300 GPa. Moreover, under a very high load, the gion. Here, we have systematically studied the behaviors of beveled anvil will “cup” (bend outward), causing one of the major three DACs with three typical culet sizes, one of which reached limitations on the achievable pressure for a beveled DAC (18). 4 Mbar. In addition, by using submicrometer synchrotron X-ray Thus, understanding the behavior of a DAC under very high probe, double cupping of beveled diamond anvils up to 4 Mbar pressures, including the process of pressure loading, the evolution is reported experimentally. Our results provide important of diamond anvil elastic deformation, gasket flow, and pressure guidance for future DAC studies to extreme high pressures. distribution along the culet area, is indispensable for further un- derstanding and developing the DAC technique. Author contributions: B.L. and H.-k.M. designed research; B.L., C.J., J.W., R.X., W.L., and Z.C. performed research; K.Y., R.X., W.L., and Z.C. contributed new reagents/analytic Some previous experimental studies explored the behavior of tools; B.L., C.J., W.Y., K.Y., J.C., and H.-k.M. analyzed data; and B.L., J.C., and H.-k.M. the DAC under high pressures (18, 19). However, detailed local wrote the paper. strain stress information (e.g., on the small culet area) could not Reviewers: T.Y., University of Tokyo; and C.Y., Washington State University. be derived due to the limitation of the probe size, which typically The authors declare no conflict of interest. ∼ μ has a 5 10- m spatial resolution. A few finite-element modeling Published under the PNAS license. studies also analyzed the anvil deformation (20, 21) and its stress 1To whom correspondence may be addressed. Email: [email protected] or hmao@ciw. state (21–23) under high pressures. For a beveled diamond anvil edu. geometry, the finite-element modeling unveiled double cupping in This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. the center flat and beveled areas at 300 GPa (21). However, this 1073/pnas.1721425115/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1721425115 PNAS Latest Articles | 1of5 Downloaded by guest on October 2, 2021 addition to the few differences between the normal and the radial stress, the force calculated using the pressure from the XRD could be a good estimation. Table 1 lists the forces on the 250- and 20-μm culet area and the average pressure, assuming the pressure is homogeneous on the whole 250-μm area at the four center pressures. At the highest load of ∼400 GPa, the force on the culet area is 4,901 N, which is almost a half-ton force on the diamond anvils with an average pressure of 100 GPa. However, when the central pressure is ∼170 GPa, the average pressure is only 11 GPa, indicating that at high loads, the cupping of the diamond anvils helps to redistribute the pressure and reduce the pressure con- centration on the small culet area. Table 1 shows the reduction in the center/average pressure ratio number from 15 to 4. The calculated forces on the whole culet area at different center pressures have also been plotted in Fig. 2A as four open circles, Fig. 1. Flat and beveled anvils (Left), a symmetric DAC (Upper Right) and showing that they follow the same trend as the real experiment. Boehler-type seat (Lower Right). (Left) Sizes of a beveled (A and B) and a flat culets (C) are indicated. Since most DAC experiments are under 100 GPa, after the study with 20-μm beveled to 250-μm culets we also performed an experiment with flat 300-μm culets to investigate their behavior A schematic of the 20-μm beveled to 250-μm anvil is shown in up to ∼90 GPa. In this experiment, a 1 × 2-μm focused X-ray Fig. 3B as a reference. In this study, the diamond anvils failed beam was used as the probe. Fig. 5A shows the evolution of the during the 2D XRD mapping at a center pressure of ∼400 GPa. pressure distributions at elevated center pressures. Straight lines Thus, only 1D XRD line-scan (1-μm step size) data were used to provide the pressure distribution at that pressure point, and subsequently fewer data points were obtained (Fig.